Cancer-related fatigue (CRF) is the most commonly reported side effect of cancer treatment and is defined by the National Comprehensive Cancer Network as a distressing, persistent, subjective sense of physical, emotional, and cognitive tiredness or exhaustion related to cancer or cancer treatment that is not proportional to recent activity. When we interviewed 200 hematopoietic stem cell transplantation (HSCT) survivors, 40% reported clinically significant CRF. Various pharmacologic agents to treat CRF have been studied but there is insufficient evidence to recommend their use. Multiple non-pharmacological interventions for CRF have been shown to have clinical benefits, however, those interventions are costly and involve significant patient burden. Our recent research provides compelling evidence that systematic light exposure (sLE) to full spectrum bright white light (BWL) yields clinically significant reductions in CRF (effect size d = 0.98), with minimal expense and low patient/survivor burden. We seek to determine the biological and chronobiological mechanisms underlying sLE's effects on CRF, which is consistent with recent NCI recommendations for high-priority research. The primary objective of this R21 is to determine the feasibility of investigatig fatigue-related biological markers (cortisol and inflammatory cytokines), hypothesized to be critical to the impact of sLE on CRF. The proposed research will address three specific aims: 1) Assess the feasibility of collecting saliva and blood samples to explore potential biological mechanisms of the effects of BWL on CRF. We will investigate recruitment, adherence, and retention of 40 autologous-HSCT survivors with CRF receiving sLE (20 receiving BWL and 20 DRL). We hypothesize that collection of biological samples, recruitment, adherence, and retention will be feasible. 2) Explore whether sLE induces changes in biological variables (i.e., cortisol and inflammatory markers) and chronobiological variables (circadian activity rhythmicity) that have been implicated as correlates of CRF. We predict that normalization of cortisol levels and decreases in the inflammatory cytokine response in those exposed to BWL compared to DRL. 3) Determine effect sizes needed to fully power a study to examine whether biological variables mediate the effect of sLE on CRF.